Kalidas, C.

Rao, V. Srinivas

English

NOPR

Indian Journal of ChemistryVol. 15A, February 1977, pp. 118-UOProtonation of p- & m-Nitroanilinium IonsDiethylene Glycol-Water Mixtures.InV. SRINIV AS RAO & C. KALIDASDepartment of Chemistry, Indian Institute of Technology, Madras 600036Received 12 February 1976; accepted 21 June 1976The thermodynamic ionization constants of p- and m-nitroanilinium ions have beendetermined spectrophotometrically at 25° in diethylene glycol and its aqueous mixtures.The pK of both the indicator ions decreases continuously with addition of diethylene ~lycol,passes through a minimum at about 80 wt % organic solvent and then increases in the puresolvent. The variation of ApK (pKs-pK",) with solvent composition was analysed in terms ofthe changes in transfer free energtes of the different species involved in the ionization processin these solvent mixtures.STUDIES on the protonation equilibria ofweak organic acids and bases in non-aqueousand mixed aqueous solvents as a functionof solvent composition have provided- useful insightto an understanding of solute-solvent interactionsin different media. Although such studies involvingni troariilinium ions have been mades in someamphiprotic solvent-water mixtures, no measure-ments have been reported in diethylene glycol orits aqueous mixtures. In this paper, which is incontinuation of our earlier investigatiom3,4 on thethermodynamic behaviour of electrolytes and ionsin these media, the ionization constants of p- andm-nitroanilinium ions are reported in diethyleneglycol and its aqueous mixtures of varying composi-tions in order to understand the solute-solvent inter-actions in these media.Materials and MethodsDiethylene glycol (BDH, LR) was purified asdescribed3 previously. The fraction boiling at120°/10 mm was collected and stored out of contactwith air. This sample had a refractive index1·4445 at 27'5°. Doubly distilled water was usedin the preparation of diethylene glycol-watermixtures.p- and m-nitroanilines, recrystallized twice fromaqueous ethanol (melting points 145° and 114°,respectively), were used in the spectral measure-ments. Hydrogen chloride used as the acid in thesemeasurements was generated and purified as des-cribed earlierli and bubbled into the desired solvent.The strength of the stock acid was determined bytitration against aq. sodium hydroxide which wasstandardized previously using a solution of potassiumhydrogen phthalate.Indicator ratios - All optical measurements werecarried out at 25° on a PMQ II Carl Zeiss spectro-photometer. The indicator ratio, CBH+/CB,was cal-culated" from relation (1)CBR+ EB-ECB = E-EBR+where EB represents the molar extinction coefficientof the indicator in the basic form, E is the molar... (1)118extinction coefficient in the experimental solution,and EBH+is the extinction coefficient of the conjugateacid of the indicator at a given waveler gth. Inthe present case, absorbance measurerner.ts weremade at the wavelength of maximum absorptionof the basic forms of the respective indicators ar.dEBR+was found to be negligible in all cases. How-ever, EBwas found to vary to a small extent withthe solvent composition and was taken into accountin calculating the indicator ratios in differentmixtures. Further experimental details are similarto those given elsewheres. The solubilities of theindicators in different solvent mixtures were deter-mined spectrophotomotrically at 25° in accordancewith the procedure given earlier?ResultsThe pK of the indicators, wherein equilibrium(2) operated,BH+ = B+H+ ... (2)was obtained by extrapolation of the plots of PK'against C, the concentration of acid, to C =t 0 inall solvent compositions. pK' is given by Eq. (3), CBH+PK = log CB-log CR+= PK +r(c) ... (3)In calculating PK', the acid was assumed tobe completely dissociated in all solvent compositiors.The plots were found to be Iiriear in all cases, Inthe case of m-nitroaniline, a correction was applied"to the concentration of the acid at equilibrium takingBH+ formed into consideration. Tne PK and solu-bility data of the two indicators are recorded inTable 1. The transfer free energies of the neutralindicators [~G~B)J in different solvent mixtures [L\G?of a species i being defined by ~G? = G?(.) - G?(II) ,where G?(s) and G?(f1J) represent the standard freeenergies of i in the solvent and water, respectively]were calculated from expression (4)~G~ = 2'303 RT log 55'" ... (4)swhere 5111 and 5. representing the solubilities of theindicator in water and solvent respectively, areRAO c\ KALIDAS: PROTONATIoN OF NITROANILINIUM IONSTABLE1 - pK ANDSOLUBILITYDATAAT25°Diethylene p-Nitroaniline m-Nitroanilineglycol°t °t(wt 'Yo) pK* Solubility x 10' -t.G(B) pK* Solubility X t03 -t.G(B)(mole kg-I) (J mole'") (mole kg:"] (J mole'")0 (0,99)' (4,379)' (2,46), (68.28)'20 0·64 15·13 3104 2·22 135·73 170340 0·57 37'70 5340 2·03 320·60 383360 0·24 146·02 8695 ·1·84 1095·80 688180 0·18 519·11 11840 1·79 4306·50 10280100 1·89 1163·67 13100 3·46 14896·00 13360•Accurate to ±0·05 unit.tAccurate to == 80 ] mole-toalso given in Table 1. The validity of Eq. (4) de-pends on the ratio of the activity coefficients of Bin water and solvent being unity ill the saturatedsolutions for which there is some evidence in theIiteraturet-". Considering the pK difference of anindicator between solvent and water, ApK (PKs-PK.). it is possible to write2'303RTAPK = AG~diSS)= AG;H+)+AG~B)-AG~BH+)... (5)where AG(H+)and AG(BH+)represent the transfer freeenergies of H+ and BH+, respectively. Eq. (5) mayalso be expressed as2'303 R.TAPK = AG('diss)= AG(HCl)+AG(B)- AG(BHCl) ... (6)where AG(HCl) and AG(BHCl) are the transfer freeenergies of hydrochloric acid and the nitroanilinehydrochloride, respectively. Using AG(H+) obtainedearlier' on the basis of ferrocene redox couplereference method, AG{'BH+)was evaluated from Eq.(5). Similarly, AG(BHCl) was calculated from Eq.(6) utiliaing the transfer free energies of hydrochloricacid in these solvents determined previously", Allthese results (molal scale) are recorded in Tables2 and 3.DiscussionThe PK values of both the indicators (Table 1)decrease continuously with the addition of diethyleneglycol, pass through a minimum at about 80 wt %organic solvent and then sharply increases inthe pure solvent. Since reaction (2) is isoelectric,change of dielectric constant of the solvent withaddition of diethylene glycol should have negligibleinfluence on this reaction. Thus, considering thevariation of AG{'diss)with solvent composition forthe two indicators in terms of the transfer free en-ergies of the various species involved in theseequilibria, it is seen from the data in Tables 2 and3 that this variation is due to two competing factors(i) the continuous increase of AG{,HCl)with theaddition of diethylene glycol which becomes very steepin the region of pure solvent and (ii) the continuousdecrease of AGoB)-AG{,BHCl)over the whole compositionrange. The increase of AG(diss) beyond 80 wt %TABLE2 - THERMODYNAMICQUANTITIESOFP-NITROANILINE(MOLALSCALE)(All values are in ] mole=}Diethy- t.Go*(HCl) t.Go(diss)t.Go(BHCl)t.Go(BH+)t.Go(B)-lene t.Go(BHCl)glycol(wt 'Yo)20 1649 -2000 545 -1744 -364940 2598 -2397 -345 -6144 -499560 4981 -4260 546 -9675 -924180 8665 -4623 1448 -13005 -13288100 24540t 5091 6349 -24625 -19449*Also applicable to Table 3.[Unpublisbcd work.TABLE3 - THERMODYNAMICQUANTITIESOFm-NITROANILINE(MOLALSCALE)(All values are in ] mole'<)Diethy- t.Go* (H+) t.Go (diss) t.Go (BHCI)leneglycol(wt 'Yo)t.Go(BH+) t.Go(B)-t.Go(BHCl)20 -640 -1373 1319 -970 -302240 -3201 -2456 1221 -4578 -505460 -5240 -3540 1640 -8581 -852180 -5788 -3824 2209 -12244 -12489100 -6434 5711 6089 -25505 -18829*Applicable also to Table 2.organic solvent is due to a larger increase of AG(HCl)compared with the decrease of AG(,B)-AG(BHCl)inboth cases.A comparison of AG(BH+)and AG(H+) for these in-dicators shows that AG{'B~+)decreases rapidly withthe addition of diethylene glycol while the decreasein 6.G(H+)is less marked under the same conditions.The negative values of the transfer free energiesot the two ions indicate that they are solvated pre-ferentially by the mixed solvent than by water.The large decrease of 6.G(BH+)with the addition oforganic solvent is most possibly due to the presenceof the bulky hydrophobic benzene rings in thenitroanilinium ions which tend to get stabilized119INDIAN J. CHEM., vOL. 15A, FEBRUARY 1977through solvation by the organic solvent than bywater. Further, the solvation of the nitroaniliniumions through the partially protonated hydrogenatoms attached to the nitrogen atom and thenegatively charged oxygen centre of the solventdipoles is also a contributing factor. This alsoexplains the relatively less positive values of AG~HCl)as compared with AG{iICl) over the whole ran=e ofsolvent composit.ions. . ~he negative v~lue.s of AGi~,)of the two nitroanilines and their mcreasingmagnitude (in the negative direction) with increas-ing proportion of the organic component can beexplained as due to the presence of the hydrophobicbenzene rings in these molecules as stated above.Thus the overall variation of APK in these solventmixtures is largely controlled by specific ion-solventinteractions as well as the basicity of the mediumindicated by the proton medium effect.120AcknowledgementOur sincere thanks are due to the CSIR. NewDelhi. for the award of a senior research fellowshipto one of us (V.S.R.).References1. BATES. R G.• J. electroanal. Chem., 29 (1960), 209.2. KUNDU, K. K., DE, A. L. & DAS. M. N .• J. chem, Soc••Pe,.lIin II, (1972), 2063.3. KALIDAS, C. & SRINIVASRAO, V .• J. cu«. Engng Data.19 (1974). 201.4. SRINIVASRAO, V. & KALIDAS, C., Indian J. Chem., 13(1975), 1303.5. KALIDAS,C. & PALIT, S. R, J. chem. Soc., (1961), 3998.6. PAUL, M. A. & LONG, F. A., Chem. Rev., 57 (1957), 1.7. CHATTANATHAN,N. & KALIDAS,C., Bull. chem, Soc. Japan,44 (1971). 1004.8. ROBINSON,R A., J. Res. Nat. Bur. Stand., 74 (1970), 495.9. SCHINDLER,P., ROBINSON,R. A. & BATES, R. G., J. Res.Nat. Bur. Stand., 72 A (1968), 141.

Protonation of p- & m-Nitroanilinium Ions Diethylene Glycol-Water Mixtures

http://nopr.niscair.res.in/bitstream/123456789/52909/1/IJCA%2015A%282%29%20118-120.pdf

Protonation of p- & m-Nitroanilinium Ions Diethylene Glycol-Water Mixtures

Abstract

Similar works

Full text

Available Versions

NOPR